(±) 2-[(Diphenylmethyl)sulfinyl]acetamide of formula 1, also known as modafinil, exerts a wakefulness-promoting effect on humans and animals.

The psychotropic activity of modafinil was demonstrated in tests on animals such as those described in U.S. Pat. No. 4,177,290 (“the '290 patent”) and in clinical trials on human patients. Modafinil racemate is approved by the F.D.A. for treatment of narcolepsy.
The '290 patent describes preparations of modafinil. In Example 1 of the '290 patent, modafinil is prepared by reacting 2-[(diphenylmethyl)thio]acetic acid chloride with ammonia, isolating the product amide and then oxidizing its sulfide group with hydrogen peroxide in acetic acid. Example 1a of the '290 patent describes a different synthetic method for an industrial scale preparation of modafinil. Benzhydrol is reacted with thiourea to form an intermediate which is then hydrolyzed to 2-[(diphenylmethyl)thio]acetic acid. The acid is then oxidized in situ with hydrogen peroxide in a mixture containing chloroacetic acid and water. The resulting sulfoxide is then treated with dimethyl sulfate to methylate the carboxylic acid group. The resulting ester is derivatized with ammonia to modafinil.
Each of these methods uses hydrogen peroxide to oxidize a sulfide group to a sulfoxide. Drabowicz, J et al. Synthesis, 1990, 37-38 describes a procedure for oxidizing sterically hindered sulfides to sulfoxides. The procedure uses hydrogen peroxide as the oxidizing agent, methanol as the solvent and a mixture of sulfuric acid and one of several branched aliphatic alcohols as a catalyst. The procedure is well adapted for oxidizing sterically hindered sulfides. No products of over-oxidation were observed by thin layer chromatography of the reaction mixtures. Use of this methodology to prepare modafinil has not been described in the literature.
Sulfides also may be oxidized to sulfoxides with other oxidizing agents, such as sodium periodate, t-butyl hypochlorite, calcium hypochlorite, sodium chlorite, sodium hypochlorite, meta-chloroperbenzoic acid and sodium perborate. March J. Advanced Organic Chemistry 1201-02 (4th ed. 1992).
We have discovered that the process of Example 1 of the '290 patent suffers from a problem of over-oxidation of the sulfide to sulphone 2.

By comparing the above presented chemical structures it will be readily appreciated that separation of the sulphone once formed from modafinil is a difficult task. Therefore, the development of selective oxidation methods are required in order to obtain modafinil free of sulphone after one or more recrystallizations.
In the process described in Example 1a, significant amounts of intermediates 2-[(diphenylmethyl)sulfinyl]acetic acid 3 and methyl 2-[(diphenylmethyl)sulfinyl]acetate 4 are obtained because of incomplete conversion of the starting materials in Steps (b) and (c). Becue, T; Broquaire, M. J. Chromatography 1991, 557, 489-494. These compounds are also difficult to separate from modafinil.

Due to the volume of solvent used by industrial scale processes and the environmental issues raised by the disposal of large amounts of organic solvent, an industrial preparation that yields modafinil essentially free of impurities and requires only one crystallization of the end product to obtain modafinil free of impurities within the limit of detection is highly advantageous over an alternative process that requires repeated recrystallizations to obtain modafinil in equivalent purity. Although Example 1 a of the '290 patent is described as an industrial process, two recrystallizations were used to obtain the product as a white crystalline powder. The composition of that powder is not reported.
It would be highly desirable to have an improved process that produces modafinil essentially free of sulphone 2 so that it may be obtained in high purity by a single crystallization. In addition, it also would be highly desirable to avoid using dimethyl sulfate, one of the reagents in Example 1a, since it is highly toxic.
While pursuing the object of efficiently producing modafinil in high purity, we discovered that modafinil can be crystallized into several distinct solid state crystalline polymorphic forms. Crystalline forms of a compound are differentiated by the positions of the atomic nuclei in the unit cell of the solidified compound. The differences produce different macroscopic properties like thermal behavior, vapor permeability and solubility, which have practical consequences in pharmacy. Crystalline forms of a compound are most readily distinguished by X-ray analysis. Single crystal X-ray crystallography yields data that can be used to determine the positions of the nuclei which in turn may be visualized with computer or mechanical models, thus providing a three-dimensional image of the compound. While single crystal X-ray studies provide unmatched structural information, they are expensive and quality data can sometimes be difficult to acquire. Powder X-ray diffraction spectroscopy is used more frequently by the pharmaceutical industry to characterize new crystalline forms of drugs than is single crystal X-ray analysis. Powder X-Ray diffraction spectroscopy yields a fingerprint that is unique to the crystalline form and is able distinguish it from the amorphous compound and all other crystalline forms of the compound.
There is a wide variety of techniques that have the potential of producing different crystalline forms of a compound. Examples include crystallization, crystal digestion, sublimation and thermal treatment. In the laboratory preparation in Example 1 of the '290 patent, modafinil is first precipitated by adding water to a reaction mixture containing modafinil, water and excess hydrogen peroxide. Modafinil is then recrystallized from methanol. In the industrial scale preparation of Example 1a, modafinil is obtained as a white powder by first crystallizing from a 1:4 mixture of methanol and water and then crystallizing again from a 1:9 methanol/water mixture. Crystallization from methanol and a 1:9 methanol/water mixture produces modafinil in polymorphic Form I. Modafinil Form I is characterized by a powder X-ray diffraction (“PXRD”) pattern (FIG. 1) with reflections at 9.0, 10.2, 11.2, 12.9, 15.2, 15.8, 16.3, 17.7, 18.2, 19.3, 20.5, 21.6, 21.9, 23.2, 26.6±0.2 degrees 2θ.
U.S. Pat. No. 4,927,855 describes the preparation of the levorotatory enantiomer of modafinil by chiral resolution of the 2-[(diphenylmethyl)sulfinyl]acetic acid with α-methylbenzyl amine. After recovery and amidation of the enantiomerically pure acid, (−) modafinil was obtained as white crystals by crystallization from ethanol.
The discovery of a new crystalline form of a pharmaceutically useful compound provides an opportunity to improve the performance characteristics of a pharmaceutical product. It enlarges the repertoire of materials that a formulation scientist has available for designing, for example, a pharmaceutical dosage form of a drug with a targeted release profile or other desired characteristic. It is clearly advantageous when this repertoire is enlarged by the discovery of new crystalline forms of a useful compound. Five new crystalline forms of modafinil that are not accessible by following crystallization procedures previously described in the art have now been discovered.